All-Optical Poling of Ultralong Silicon Nitride Spirals

Integrated silicon nitride (Si3N4) platform is arguably one of the most established platforms for third-order nonlinear photonics (χ(3)) [1,2]. Recent advancements have addressed its absence of second-order nonlinearity (χ(2)) via all-optical poling (AOP) [3]. While ultra-low loss Si3N4 has been recently leveraged to demonstrate χ(3) parametric gain [4], still scalability of AOP in such long waveguides remains uninvestigated. Here, we study AOP in Si3N4 spiral waveguides, analysing how design impacts the efficiency and bandwidth of the poled waveguides. The samples used in this study were fabricated using photonic damascene process [5]. The waveguides feature spiral configurations, including circular bends [4], Euler bends [6], and Archimedean spirals. Mode mixing significantly impacts AOP in waveguides, by limiting power in a given mode. It clamps the efficiency in grating inscription, influenced by the linear mode overlap in regions with different curvatures. To optimize this overlap, the curvature must change adiabatically. Euler bends begin with zero curvature, leading to smoother transitions and less mode mixing than spirals with circular bends. Because Euler bends also have a lower maximum curvature, they typically exhibit reduced bending losses compared to Archimedean designs.